New principles of greenhouse crop management have emerged from Dutch horticulture industry experts and scientists from Wageningen University, a Netherlands institution recognized for its agricultural science program. Hundreds of Dutch growers have been trained in greenhouse climate control practices called Growing by Plant Empowerment (GPE), fundamentals of which have been outlined in a 2018 book, “Plant Empowerment” and in related online tools at Letsgrow.com.
The methods presented in the book diverge from our understanding of traditional greenhouse climate control. They remind us that plants are physical objects subject to the laws of thermodynamics in addition to being biological organisms.
Rather than focus climate control on static air temperature and humidity or vapor pressure deficit (VPD) targets, GPE controls the growth and flowering processes based on three balances: the energy balance, the water balance and the assimilates balance. Assimilates are the sugars made during photosynthesis that are used for growth. The three balances are managed simultaneously and kept in equilibrium by the tiny pores in leaves called stomata that allow water vapor to be evaporated and carbon dioxide to be absorbed. In effect, GPE is about managing the stomata rather than the environment per se, keeping the pores in open position to maximize photosynthesis and evaporation.
GPE emphasizes plant health, fruit or flower quality, and resistance to stress and disease. This results in predictable yields despite the dynamic nature of greenhouse environments. Energy sav
GPE in a nutshell
Greenhouse control is based on supporting the three plant balances rather than conditioning the air to certain setpoints. Instead of a fixed temperature regime, setpoints are adjusted according to predicted daily light integral (DLI) to create a more constant ratio of temperature to radiation to balance growth.
The emphasis is placed on growing at warmer temperatures. During sunny periods, rather than increasing ventilation or mechanical cooling, temperature and humidity are allowed to increase in the greenhouse to keep stomata open for CO2 absorption, maximizing photosynthesis. Under high light and enriched CO2, most plants’ optimum temperature is 86°Fahrenheit, according to the white paper “Next Generation Growing: Plant empowerment and plant balances.”
The “Plant Empowerment” book’s authors use the term evaporation to encompass both water transpiring from stomata and evaporating from micropores in the leaves, and the GPE methods seek to avoid interruptions in this flow of water vapor from the plant. Heating pipes or warm air currents are used in the absence of light at night. To keep up with water demand, irrigation is triggered based on all energy flows (light, heat, convection, evaporation), rather than just light. This emphasis on evaporation is to keep water and mobile nutrients such as calcium flowing to the plants’ growing points.
The authors also state that without night ventilation, plant cells can be damaged by root pressure (water turgor) building up, creating sites for possible fungal infection. Diseases seldom occur due to poor climate conditions exclusively, they report, but rather because the sub-optimal conditions combined with disturbances in the plant balances lead to lower resiliency. For example, disease is prevented by avoiding condensation that occurs when leaf temperature drops below dew point. This is accomplished with thermal screens to block radiation of heat energy from the plant to a colder object, such as the greenhouse roof. This method is based on the law of conservation of energy, which states that energy can’t be created or destroyed. It can only be converted into another form of energy.
Maintaining a balance of photosynthetic assimilates in the plant is paramount in the GPE methodology. Photosynthesis increases along with light radiation up to a point called the light saturation point. Under these conditions, photosynthesis can be enhanced by increasing humidity to keep stomata open for CO2 absorption. According to GPE, rather than cooling the greenhouse, temperature should be allowed to rise to speed the biochemical reactions of photosynthesis, as long as water stress can be avoided.
But this is only one half of the balance. Now that the plant is maximizing creation of assimilates, the goal is to use these sugars immediately for growth, flowering or root production, rather than convert them to starch for storage. Accumulation of starch in the leaves can slow photosynthesis by regulating enzymes. Some sugars might also be converted to cellulose for heavier stems and leaves, which provide no value.
In essence, GPE calls for the use of light to make sugar and heat to forge that sugar into new material. On the other hand, growers don’t want this high-temperature regimen during dim conditions, as the plant may need more assimilates for “everyday maintenance” of the leaves and cellular apparatus than the plant can make under low light, resulting in cessation of growth and lack of resilience.
GPE provides proactive methods that render more predictable, uniform yields by maintaining a more constant ratio of temperature to radiation. First, environmental control systems with photosynthetically active radiation, or PAR, sensors can track and estimate DLI, and growers can program temperatures to then increase with increasing predicted DLI. Additionally, night temperatures can be adjusted to dial in the proper 24-hour temperature once that day’s DLI has been locked in. Determining the target temperature follows the formula: Target Temperature = 18 + (2 x DLI/10) with 18 being the base temperature in Celsius for a dark day. A more typical DLI of 40 for cannabis would have a temperature of 26°Celsius (78.8°Fahrenheit). The GPE authors give examples of when to adjust this formula for challenging conditions. For example, in very hot climates or seasons, the baseline temperature of 18°Celsius could be shifted upward to 20°Celsius to account for the difficulty of cooling the greenhouse environment and to lessen fluctuations when cooling cycles on and off. (Note: This same formula might prove useful for determining DLI targets as you lower temperatures near harvest.)
It is also important to consider the plant load, which is defined as the number of plants per square meter and the number of flowers and branches per plant. Can the assimilates the plants are producing actually sustain the number of flowers you hope to yield? GPE principles suggest that it is best to use low plant loads with high temperature regimes to maximize quality and produce predictable, uniform yields. Higher plant loads would require lower temperatures, negating the benefits of the higher temperatures described previously. If both plant load (flower canopy) and temperatures are high, growth and flower yield may be diminished due to competition for assimilates for maintenance.
As far as equipment, assimilates production also can be improved by increasing light or light interception, using more lights, intracanopy lighting, light-diffusing roof and wall panels, or light-diffusing shade screens inside the structure. Diffusing light improves penetration into the lower foliage, according to a 2015 research paper published by Frontiers in Plant Science. This prevents lower leaves from turning from sources of photosynthetic assimilates to “sinks” (organs that use them up).
The “Plant Empowerment” authors also discuss “energy balance,” which refers to the four different types of energy flows in your greenhouse: light, heat, convection by air currents and evaporation. These four energy flows can be measured, and their values must add up to zero, according to the law of conservation of energy. Plants can’t make their own light (that we can observe easily) or heat, so these can only be energy…
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